Linked objects reflect black. Is it a 2.5 bug?

I don’t know, what else to do: I tried about everything I could think of but to no avail. In my current project all the objects are linked from other files, where I model and texture them. Now, I wanted to add a mirror, but some objects reflect just fine and others are just black. Don’t know what’s going on. I’m using the compositor, but nothing too fancy there. anybody got any idea what the problem might be?